5th generation civic hatchback - improving aerodynamics

[avitet]

Originally Posted by AndrewJ

So, I just did my first long trip with Harold yesterday, I drove from McKinleyville to San Fransisco, a distance of about 275 miles. It wasn't a big deal until I got to the mountanous portion of the trip. After an elevation gain of 1800 feet in 20 miles or so, the car started acting kinda funny.

The temp gauge was showing normal temps, but the motor wasn't making enough power to climb the steeper grades. The only thing that seemed to help was kicking the heater into high gear, sucking some heat out of the engine bay.

I think I'll be cutting out a vent in the airdam for the return trip. That'll give me an opportunity to rig up a variable open-close vent using a manual choke cable. I've kinda been wanting to try that anyway.

But before I do that, does anyone have any ideas as to what would cause the motor to loose power in that situation? The coolant temps weren't abnormally high, but due to the high load and low speed it's not unreasonable to think that under-hood temps were higher than normal.

Oh, and by losing power I mean that I was unable to keep up a "reasonable" speed (50mph) even after downshifting into 4th. By downshifting to 3rd I could keep about 42mph.
I can keep 50mph on my "calibrated" test hill all the time (it's a similar grade to the hills I had trouble with), though that's the only hill on a 10 mile drive.

assuming you have a 1.5 which makes 70 hp at sea level you only have 33 at 1800 ft sorry if someone else has already made this statement i havent reached the end of the thread yet http://www.webcalc.net/calc/0757.php

[Daox]

Are you planning on having an onboard charger so you can just plug in when you get home?

[AndrewJ]

Originally Posted by reformed

I've read on here that intake air temps of above around 110 can possibly hurt FE. I know nothing of this, so I'm curious if you had any results from the WAI alone. I actually just flipped my "intake" (the little endcap with the square on the one end) to the way you have it facing but I havent run any duct work yet.

Yeah, I've heard the same kind of thing... however.... I've also done some reading on the interwebnets that suggests that high IAT's aren't really bad at all (except for making power)
I've only really looked at info about Honda IAT's, but I believe that there was a thread on GS about putting resistors on the IAT to "trick" the computer into reading REALLY high IAT's (like in the 220-240 range) with no ill effects.
Basically, the effects of the resistor mod netted gains for some, and a wash for others.

As for my HAI (I'm calling it an HAI from here on out because it's normally 140º or above, and that's pretty hot) I think it has definitely contributed a bit to my mileage. My last tank jumped up 2mpg, and the new mods during that tank were the HAI and the partial bellypan with trimmed airdam.

I don't think that 2mpg is attributable to the bellypan/airdam changes because I would have had to notice a significant increase in coastdown ability, and I haven't.

So, that leaves the HAI to attribute it to, in the form of reduced pumping losses by opening up the throttle more for the same power. I definitely noticed that this was the case after installing the HAI. I now have to press the gas pedal down significantly further to get the same power out of the engine.

Originally Posted by avitet

assuming you have a 1.5 which makes 70 hp at sea level you only have 33 at 1800 ft sorry if someone else has already made this statement i havent reached the end of the thread yet http://www.webcalc.net/calc/0757.php

Wow, didn't realize there would be that big of a disparity with elevation gain.

Well that explains quite a bit


And Daox:
Yeah, a battery charger is in the works. In transit to be specific. I got a cheapie smart-charger off of the ebays. I couldn't find a decent one around here for less than $100 so I have to wait on shipping from Indiana...


Edit:

Back to the horsepower loss thing. Anybody know at what elevation manufacturers rate their engine's horsepower at? Seems like it would prolly be sea-level, but damn would all those sports-car buyers in Denver be getting the shaft.

[MetroMPG]

I was shocked by the power difference that elevation calculator shows (enough that I'm skeptical about its accuracy to be honest). It's immense - and should permit people at higher elevations to get noticeably better fuel economy.

[mimsmetro]

Your work has inspired me.

thank you.

[reddago4]

The horsepower loss with elevation must be wrong I think.
I put in 96hp at sea level and calculated for 5000ft (Denver) and got a hp loss of 144. so I guess that means that I would have a negative 48 hp. I wonder what that would do for economy?

[LostCause]

Using an ideal thermodynamic cycle, backward engineering the engine efficiency using factory specs, and adjusting for 5000ft., I got a hp shift from 92 hp to 83 hp.

Assumptions made were standard temperature & pressure, no manifold vacuum, engine efficiency of 45% (admittedly high) and a constant peak HP rpm of 5600 (factory specs) for changes in altitude. I believe engine efficiency is off due to manifold vacuum assumptions, but the change is still relevant

If anyone wants the theoretical method for their own calculations, just ask. If not...an easier approximation is HP = (altitude density / sea level density)^1/2, but I'm not 100% sure on that one. In any case, good night.

- LostCause

[avitet]

sorry for the for the bad forumla it dose appera to be incorrect here is one that shoul be more accurate http://wahiduddin.net/calc/calc_da_rh.htm and a chart for air pressure at various altitudes http://www.engineeringtoolbox.com/ai...ure-d_462.html

[AndrewJ]

Thanks for the clarifications guys!


Edit:

P.S. - Filled up with gas today for $3.99/gallon for regular unleaded


P.P.S. - I think somethin's up with the alt-kill switch. It may not be "killing" the alternator completely. No matter the battery voltage when the car is off, when running and alt-killed the voltage hovers in the 12.3-12.7 range.
I even tried running the headlights, rear defroster, fan, radio, etc.... No voltage dip. I'll just have to run it killed for a few days and see what happens.

[AndrewJ]

So I filled up the tank today for a whopping 49mpg.


Dissapointing to say the least, especially for the first full tank with the alternator-switch. The alternator was switched "off" for about 80% of the tank.


I think that the alternator is never really getting turned completely "off" but rather the switch keeps the alt from jumping up to it's charging voltage of about 13-14 volts.

So, even with the alt-charging switch turned "off" the alt is still charging at 12.5ish volts drawing power. And when it's turned back "on" to 13.5 volts it draws even more power to charge....

So I'm going to start unbelting the alt completely when I can. We'll see if that helps this tank out at all.


Oh, and I weeded out the 2 duds from my 4 batteries. I charged each of them up with my new 12v charger (which rocks btw) and then hooked them up with jumpers to the car and ran the high-beams for 5 min, recording the voltage every 30 seconds or so.

My best battery levels out at 1 minute at 12.1 volts.
My worst battery sank below 11.6 volts within 45 seconds.

And my second worst battery became my best battery after my charger automatically went into de-sulphation mode